Control circuit for feedback, motor-operated valve

ABSTRACT

A control circuit for limiting the current flow to a direct-current driven motor, to reduce current spikes at motor start up, including a voltage-regulated, current-passing device for passing current from a direct voltage source to the motor, the device having a default configuration such as to permit full current flow there-across when the voltage is initially applied, a voltage drop device interposed the voltage source and the voltage-regulated, current-passing device responsive to the current flow to the motor and a variable-voltage output device responsive to the voltage drop developed in the voltage-drop device for providing a subsequently changing voltage for input to the current-passing device for limiting the current flow therethrough following initial application of the voltage.

This application is a divisional application of Ser. No. 08/177,062,filed Jan. 3, 1994, now U.S. Pat. No. 5,461,290, issued Oct. 24, 1995.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the field of flow-control devices. Moreparticularly, it relates to motor-driven control valves andcorresponding control circuit for use with them that combines the uniqueconcepts of limiting the current flow to the valve drive motor toovercome stiction, providing for instantaneous braking, and utilizingcontrolled rotation of the motor for precise valve positioning. Damageprevention is obtained by elimination of over-rotation thereof throughutilization of a normally open limit switch, limiting the motor drivegross movement of the valve stem.

2. Description of the Prior Art

Many processes involve the flow of liquids and gasses in tubes and pipesand require controlled regulation. Needle valves, regulating valves andshut-off valves are examples of controlled regulation used to controlthe flow of these materials. In multiple valves or remote locationapplications, the industry adopted a convention whereby motors drive thevalves from fully open to fully closed positions and to desiredpositions therebetween. The motors used in the industry includeelectric, stepper and pulse motors and the means to control them varyfrom simple on-off switches to feedback mechanisms coupled to computercircuitry.

An electric-drive motor, usually a direct current driven motor of thereversible type, is connected to the valve stem that protrudes from thevalve body. Control means are attached to the motor to provide positiveor negative direct current to drive it and the valve stem in onedirection or another. The motor is usually small, having very high speedoutput, usually in the 4,000 to 6,000 rpm range. The motor is connectedto the valve stem through a transmission that gears the high incomingrpm down to a very low range of 10 to 20 rpm. Certain problems havedeveloped in this field and have not yet been solved so that the fullutilization of motor control has not yet occurred.

For instance, one problem concerns the application of motor control tovalve closure. Through repeated opening and closing of the valve, thevalve seat wears, thus making the valve element that closes against thevalve seat travel further into the valve housing. Since virtually allvalve elements advance toward and away from the valve seat through screwthreads, the wearing of the valve seat requires the valve element andstem extending therefrom to close against the valve seat atprogressively different angular positions. This means that the valvecannot be predicted to close at any particular angular position of thestem because the slightest wear on the seat will prevent the angularposition from insuring that the valve is closed. When this occurs thevalve will leak.

In the prior art, valves are set to be closed by ordering the drivemotor to turn the valve stem until it stops turning, i.e., has forcedthe valve element fully against the valve seat. Too little motor powerwill not ensure a fully closed valve and too much motor power may causethe valve element to mash hard against the valve seat, causing increasedwear in the valve or damage to the transmission gears and othercomponents. To avoid these situations, the prior art has established thepractice of sizing the drive motor to stall at the maximum friction loadneeded to just close the valve. In other words, the motor will justclose the valve and remain in a stall condition to hold the valve shut.This convention wastes electrical power during extensive valve-closureperiods, and causes wear on the motor and drive gears in the form ofvibration, called "chatter". Furthermore, should a power failure occurduring this valve-closure hiatus, the drive motor would cease itselectrical stall and possibly allow the closed valve to drift open andallow undesired backflow.

Also, there is the problem known as "stiction". This term comes aboutbecause of frictional buildup in the valve. While the valve stem is inmotion, there is generally constant friction encountered in the valveand the load on the drive motor remains relatively uniform. That is tosay, there is no buildup of forces in the valve itself and the movementfrom full-open to nearly full-closed position may be handled by thedrive motor without difficulty. However, when the valve reaches thefully-closed position, a sudden increase in frictional load occurs inthe valve stem because of tightness achieved between the valve parts aswell as some friction buildup caused by flow interruption in the line.To open a fully-closed valve, therefore, requires the drive motor toinitially overcome this rather large frictional force or "stiction".Once the valve is cracked open by the drive motor, the stem frictiondrops to the relatively low value throughout the remainder of valvetravel. With the prior art drive motor at stall condition, holding thevalve closed, there is not enough additional power during reverseoperation to overcome this stiction and the valve often remains closeduntil movement is started by hand.

In my previously issued U.S. Pat. No. 5,137,257, I have disclosed andclaimed a means freely rotatable with the motor drive shaft forproviding a controlled amount of overturning of the motor-drive shaftfollowing full closure of the valve element against the valve seats andsimultaneously storing a portion of the drive energy expended in theoverturning to bias the valve in its closed position, and to dischargethe stored energy to aid in opening the valve upon reverse turning ofthe drive motor and motor-drive shaft.

However, further problems have been determined to exist in theday-to-day operation of such a valve. Because of the high rotation speedof the motor and its sudden start-stop operation, the associated gearstend to wear and deteriorate over time, causing motor failure.Additionally, when electrical energy, in the form of DC current in theamount of 12 or 24 volts, is applied to the electric drive motor, thereis a surge of current that passes into the motor and through thecommutator that eventually subsides to an amount of currentproportionate to the load on the motor. During this initial surge, or"spike" as it is known in the trade, arcing occurs between the motorbrushes and the commutator such that, over a period of time, thecommutator shows signs of wear and erosion and eventually fails, therebyrendering the motor unusable. Because the motor is so small and furtherbecause it is made by high-speed production techniques, the cost ofrepairing the motor is significantly large compared with the initialprice thereof. It is not uncommon for motors, such as those presentlyused in motor-driven valves of the type herein described, to have auseful life limited to about 2500 hours.

Another significant problem exists with respect to the motor continuingto turn after power has been terminated thereto. The transmission,including the numerous gears, cause a significant load to the motor. Themotor spins at very high rpm to drive the transmission that, in turn,drives the needle valve to its various controlled positions. The highrpm of the motor develops a significant inertia. The inertia does notallow the motor to stop immediately upon command. The motor continues torotate powered by the inertia. Thereby the valve is turned beyond thepoint at which the controller signals a cessation of motor drive and theresulting valve position is not where it should be. When this happens,the controller must order the motor to reverse direction and bring thevalve into proper position. This extra movement of the motor results inmore wear which is a factor in early motor replacement. Finally, a limitswitch is often placed in series with the valve stem. This limit switchoperates independent of the controller and it establishes the pointbeyond which the valve stem is not to be turned, so that valve damage isheld to a minimum. In the prior art, the limit switch is generally anormally-closed single pole switch, carrying current therethrough. Suchuse of the limit switch requires it to shoulder the burden of handlingelectrical current throughout the valve travel. Such use reduces thework-life of the limit switch. Its failure to continually carry currentto the motor results in positioning the valve at undesired settings anddisrupts the process in which the valve is an important part.

SUMMARY OF THE INVENTION

This invention is a control circuit for use with motor-driven valveassemblies that solves or at least reduces the aforesaid problems tomanageable proportions. The invention includes a current-limitingfeature that substantially reduces the current surge that occurs to themotor during its start up condition. This has the effect of extendingthe life of the motor up to two hundred percent. In addition, thecontrol circuit contains a feature for instantaneously braking orstopping the rotation of the drive motor to more precisely locate thevalve at the desired point called for by the controller. Still further,this novel control circuit contains a normally open limit switch,closing upon reaching one of the limits preset in the device. Thisaction reduces the load on the limit switch and greatly extends theoperative life of the switch. Still further, this invention provides anovel concept to remove the problem of "stiction". This invention isapplicable to motor rotation in both the forward and reverse directions.

The current-limiting and surge-reducing feature of the control circuitgenerally comprises a voltage-regulated, current-passing means forpassing current from the direct voltage source to the motor, having adefault configuration such as to permit full current flow there acrosswhen the voltage is initially applied; voltage-drop means interposed thevoltage source and the voltage-regulated, current-passing meansresponsive to the current flow to the motor, and, variable-voltageoutput means responsive to the voltage drop developed in thevoltage-drop means for providing a subsequently changing voltage inputto the current-passing means for limiting the current flow therethroughfollowing initial application of the voltage. By limiting the current tothe motor when it is turning the valve in the forward direction,such aswhen the valve is driven closed, compared to the current to the motorwhen it is driving the valve in the reverse direction or opening it,provides more torque for opening than for closing and overcomes"stiction".

The instantaneous braking action of the control circuit generallycomprises a means for storing a voltage during circuit operation,including a switch element for deactivating the voltage storing meansfrom the stored voltage, voltage-regulated, current-passing means forpassing current from the direct-voltage source to the motor, anormally-open, voltage-actuated switch interconnected the motorterminals to ground, and normally-closed, voltage-regulated switch meansheld open during application of power to the motor whereupon terminatingdrive voltage to the motor causes the normally-open, voltage-actuatedswitch means to close and release the stored voltage simultaneously tothe switch element, to deactivate the current-passing means, and toclose the normally-open, voltage-actuated switch means and ground themotor terminals and the motor to immediately drain all forward andreverse EMF to ground stopping rotation of the motor.

The switch-limiting feature of the control circuit generally comprisesvoltage-regulated, current-passing means for passing current from thedirect voltage source to the motor, the means having a defaultconfiguration such as to permit full current flow there-across when thevoltage is initially applied; a normally-open, voltage-actuated switchinterconnecting the motor terminals to ground; and, a normally-open,limit-switch interconnecting the voltage source, and thevoltage-regulated, current-passing means, and the normally-open,voltage-actuated switch means that remains open during travel of thevalve between limits physically established in the assembly so that,upon reaching one of the limits, the limit switch closes to providevoltage to the voltage-regulated, current-passing means toinstantaneously stop current flow there across from the source to themotor and voltage to the normally-open, voltage-actuated switch means toclose the switch to immediately drain all forward and reverse EMF toground and stop rotation of the motor.

Each of these features may be combined separately with the feedback,motor-operated valve set forth in my previous patent, or may be appliedto other types of direct-current, motor-driven-valve, assemblies. Themaximum benefit, however, is achieved when the current-spike limiting,stiction reduction, instantaneous-braking and limit-switching featuresare all present at one time in the control circuit.

Accordingly, the main object of this invention is to provide a controlcircuit for use with a motor-driven, valve-positioning device thatrestricts the current surge that exists or is developed when current isfirst applied to the drive motor. Other objects of the invention includea control circuit having a feature of instantaneously braking therotation of the drive motor to more precisely locate the valve at theposition called for by the controller and a control circuit thateliminates stiction in the valve. A still further object of theinvention is a control circuit containing limit-switch circuitry whereinthe limit switch is in the normally open position and only closes whenit is required to stop the drive motor to prevent the valve fromoverturning beyond a fixed or desirable point.

Other objects of the invention include a control circuit that is of suchsmall size that it can be placed in a container along with the drivemotor, the transmission, the limit switch and the means previouslydisclosed and claimed in U.S. Pat. No. 5,137,257, for providing acontrolled amount of overturning of said drive-motor shaft followingfull closure of the valve element against the valve seat.

These and other objects of the invention may be observed by reading thefollowing description of the preferred embodiment in conjunction withthe drawings appended hereto. The scope of coverage sought by theinventor may be gleaned from a close reading of the Claims that concludethis specification.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view of the typical feedback,motor-operated valve to which the control circuit is applicable;

FIG. 2 is a side elevation view of the same embodiment shown in FIG. 1showing more of the frame upon which the motor-operated valve and othercomponents are mounted;

FIG. 3 is a schematic diagram of the preferred embodiment of the controlcircuit for use herein; and,

FIG. 4 is a graph showing the current-limiting feature of this inventivecontrol circuit compared to the current wave form when the circuit isnot used.

DESCRIPTION OF THE PREFERRED EMBODIMENT

This control circuit is designed for use with a valve drive assembly 1as shown in FIGS. 1 and 2. Typically, assembly 1 is attached to a needlevalve assembly 3 that comprises a valve casing 5, having a valve inlet 7and valve outlet 9 separated by a valve seat 11 operably positionedtherebetween. A valve stem 13, having a conical head machined thereon,is threadably positioned in the casing for screw-like movement into andout of sealing engagement with the valve seat. Valve stem 13 extendsoutward from case 5 for actuation from fully-closed to fully-opened andto various positions therebetween. Packing (not shown) is usually placedabout the valve stem to prevent leakage from the valve means.

As shown in FIGS. 1 and 2, an electric drive-motor 15 is provided thatreceives power through a cable 17 and is interconnected through a driveshaft 18 to a gear reduction transmission 19 to an output shaft 21.Motor 15 is generally driven by 12 volts DC power and turns upwards of6-8,000 rpm. Gear reduction transmission 19 gears down or reduces thatspeed to the output shaft speed of approximately 20 to 40 rpm,terminating in a drive gear 23. A frame 25 is arranged about thecomponents to stabilize and mount them in appropriate location. Frame 25includes a frame base 27, extension bolts 29 that are threadably mountedtherein and a plurality of spaced-apart mounting plates 31, attached tosaid extension bolts, on which the various components are mounted. Drivegear 23 is meshed with a driven gear 35 being essentially of the samesize and number of gear teeth. As arranged, and with drive gear 23 anddriven gear 35 being the same size and having the same number of gearteeth, one turn of motor-driven output shaft 21 results in one turn of adriven shaft 22, and, through a coupling sleeve 36, one turn of valvestem 13. Usually, motor driven valves of the type shown here requirenumerous turns of the valve stem between opened and closed positions yetare substantially linear with respect to the turns per degree of openingof the valve.

An electro-mechanical feed back means 37 is provided and operablyconnected to either transmission output shaft 21 or driven shaft 22. Asshown in FIGS. 1 and 2, means 37 is connected to driven shaft 22 andcomprises a potentiometer in the form of a cylindrically-shapedresistance body 39 having a movable internal wiper (not shown) actuatedby a turnable shaft 41 extending from said body. The total resistancemay be on the order of, for example, 500 ohms, and taking, for example,ten turns of shaft 41 to proceed from zero to 500 ohms. Thepotentiometer usually has three electric taps, as shown. Other types ofelectro-mechanical feedback devices are just as useful herein, such asvariable inductors, mechanical-to-electrical transducers and encoders,and all are fully contemplated for use herein within the scope andspirit of the invention.

An electric signal in the form of a certain amount of voltage, such as12 volts DC, is sent through a balancing amplifier unit (not shown), viacable 17 to two of the taps calling for the potentiometer to produce acertain voltage at the third tap. Should drive motor 15 not be in aposition to provide the appropriate output signal or voltage, currentwill be applied to motor 15 to turn it one way or the other to causepotentiometer shaft 41 to move the slide to a position where the propersignal is outputted so that the amplifier will become balanced and themotor ordered to stop. In this manner, the input signal to feedbackmeans 37 controls the direction and distance motor drive shaft 18 willmove valve stem 13 toward and away from valve seat 11. A pair of limitswitches SW1 and SW2 are positioned above feedback potentiometer 39 andare manually set to prevent driven shaft 22 from turning valve stem 13beyond its maximum opening and beyond its full closing or, if theoverturning means of my previously issued patent is used, then toprevent it from overturning beyond the angular rotation established forthe overturning.

The preferred embodiment of the control circuit, shown in FIG. 3, ismounted on a circuit board 45 and positioned above all the components.The essential elements of the circuit will be hereinafter explained morefully. A cover 47 is provided (shown in dotted outline) to encase all ofthe aforesaid components and is defined by a lower marginal edge 49 thatfits into a recess 51 formed in frame base 27. Cover 47 keeps dust, dirtand other potential invasive materials from entering the area about thevarious components and maintains a clean environment in which each ofthe components may operate.

Referring now to FIG. 3, the circuit of this invention is convenientlycontained on circuit board 45 on which are mounted a series ofcomponents including diodes, resistors, capacitors, transistors,including field effect transistors, and switches. The solid linesbetween components refer to conductors and will not be individuallynumbered except where necessary. Where conductors cross and theintersection is marked with a dot or period, it is a junction betweenthem; where one conductor crosses another and the intersection has nodot, there is no junction. The small capital letters next to linesterminating at a components, such as E, B, C, G, S, and D representsingular subcomponents of the component such as Emitter, Base,Collector, Gate, Source and Drain. Transistors and fets (a type oftransistor known as a Field Effect Transistor¹) are marked with a numberbeginning with "Q"; resistors with a number beginning with "R";capacitors with "C"; diodes with "D"; and, switches with "S". This iscommon in the art.

As shown in FIG. 3, pins 53 and 55 indicate the input of a voltage suchas 12 volts DC power to the circuit. The plus and minus signs adjacentpins 53 and 55 indicate that the potential between said pins may be, forexample,+12 volts or-12 volts. This is necessary to drive motor 15 in aforward (positive) direction or a reverse (negative) direction. Thecircuit shown in FIG. 3 contains, in the bottom half, an iteration ofthe components shown in the upper half. This iteration allows thecircuit to drive motor 15 in both forward and rearward directions.

To describe the current-limiting feature of the circuit, avoltage-regulated, current-passing means 57 is arranged between thevoltage source, pin 53, and motor 15. Its use is to pass current frompin 53 to motor 15 and it has a default configuration such as to permitfull current flow when voltage is initially applied to pin 53. It ispreferred that means 57 comprise P-channel MOSFETS Q1 and Q3. A voltagedrop means 59 is interposed pin 53 and means 57 that, as will be shown,is responsive to the current flow to the motor. It is preferred thatmeans 59 comprise current-limiting resistors R1 and R3. Avariable-voltage output means 61, responsive to the voltage dropdeveloped in voltage-drop means 59, provides subsequent changing voltagefor input to current-passing means 57 for limiting the current flowtherethrough following initial application of voltage. It is preferredthat means 61 comprise PNP transistors Q2 and Q4.

To further illustrate the operation, an input voltage of 12 volts DC isimpressed at pin 53 so that it is "high" while pin 55 is "low" or atground. This is common parlance in the art. When said voltage is appliedbetween pins 53 and 55, the same positive voltage is applied directly tothe emitter(E) of transistor Q2 and simultaneously to current-limitingresistor R1. Q1 is connected through its source(S) downstream fromcurrent-limiting resistor R1 wherein its drain(D) is connected to pin 63at motor 15. The base(B) of transistor Q2 is interposed current-limitingresistor R1 and the source of MOSFET Q2. At the initial impression ofvoltage to pin 53, the emitter-to-base potential of transistor Q2 isessentially zero (known as "reversed biased" in the art) so that thecollector(C) of Q2 does not receive any voltage. The collector at Q2 isconnected to the gate(G) of MOSFET Q1 and, with no voltage during theinitial impression of positive voltage to pin 53, the voltage at thegate of MOSFET Q1 is approximately zero. As is known in the art, aP-channel MOSFET having a positive source-to-gate potential (×12 voltsin this case) is said to be "forward biased" or turned"on" and allowspassage of full current between the source and the drain so that thefull 12 volts is applied directly to motor 15. At this time directcurrent also begins to flow from pin 53 through current-limitingresistor R1 and across the source-drain of MOSFET Q2 directly to motor15 to power it in a forward motion indicated by the solid arrow.

Current limiting now begins. As voltage is applied to the source ofMOSFET Q1 turns on, current begins to flow through current-limitingresistor R1 creating a voltage drop there-across. The voltage dropacross said resistor causes a positive potential to develop between theemitter and base of transistor Q2 ("forward biasing"). As in all PNPtransistors, when it is forward biased, it is turned "on", allowingcurrent to begin to flow from the emitter to the collector in Q2. Aresistor R2 interconnected the gate in MOSFET Q1 and pin 55 receives thepotential developed at the collector and develops a voltage dropthere-across. This voltage appears at the gate in MOSFET Q1 reducing thesource-to-gate potential. As in all P-channel MOSFETS, lowering thesource-to-gate potential causes a reduction in current flow through thesource-drain of the MOSFET (it is said to begin to "turn off" or moveinto "reverse bias") so that Q2 begins to limit the current to motor 15.As the current flow is reduced, the voltage drop across current-limitingresistor R1 decreases thereby reducing the forward bias of transistor Q2and accordingly decreasing the current flow through the collector of Q2.The voltage at the gate of Q1 is correspondingly reduced thus increasingthe source-gate voltage drop and increasing its ability to pass currentfrom the source through the drain. The dynamics of these two activities,i.e., the current-passing ability from the source to the drain of MOSFETQ1 and the voltage collected at the collector of transistor Q2 thatfeeds the gate of MOSFET Q1, operate to depress the current spikenormally developed at motor 15 when it is impressed with the initialvoltage.

Referring to FIG. 4, the solid line indicates the current and its spikedeveloped when the current-limiting control circuit of this invention isnot used with the motor herein. The current-limiting feature of thecontrol circuit shows a different wave form in dotted lines and shows asignificant reduction in the current spike. The uncontrolled currentspike has led to the limited useful life of many motors used in thistype of situation.

When the input voltage of 12 volts DC is applied to pin 53, pin 55 islow or at ground potential and diodes D1 and D8 are reverse biased whichturns off the complete bottom half or iteration portion of the circuitshown in FIG. 3. This half of the circuit becomes active when the inputvoltage is applied to pin 55 and it becomes high while pin 53 becomeslow or ground through forward biased diode D8. The only differencebetween the upper and lower halves of the circuit is the polarity of thevoltage applied to motor 15 which controls the direction of rotation. Inother words, for a reverse impression of polarity, pin 55 would be highand a voltage would immediately be impressed across current-limitingresistor R3, the source to drain of P-channel MOSFET Q3 to pin 65 on theopposite side of motor 15 from pin 63 so that motor 15 would turn in theopposite direction as indicated by the dotted arrow. The same functionsof P-channel MOSFET Q3 and PNP transistor Q4 will result, with itsemitter, base and collector and the development of a voltage drop acrossresistor R4 to the gate at MOSFET Q3.

By setting the resistance of R1 to a larger value than that of resistorR3, a novel concept is developed to overcome "stiction". The largervalue of R1 means that current to pin 63 on motor 15 is less, fordriving the valve closed, than the current passed to pin 65 to drive thevalve open. As long as the current passing through resistor R1 providesenough torque to drive the valve fully closed, then the increasedtorque, from the greater amount of current passing through R3, willovercome "stiction" and open the valve.

To describe the dynamic braking feature of the circuit, a means 67 forstoring a potential during circuit operation is provided, along withvoltage-regulated, current-passing means 57. It is preferred that means67 comprise capacitors C1 and C2 although a battery or other voltagesource is also useable therein. A normally-open, voltage-actuated firstswitch means 69 is also provided for isolating the stored potential fromthe rest of the circuit during operation of drive motor 15, along with anormally-open, voltage-actuated actuated second switch means 71interconnected first switch means 69, potential storing means 67 andcurrent passing means 57. It is preferred that first switch means 69comprise PNP transistors Q6 and Q7 and second switch means 71 compriseN-channel MOSFETs Q5 and Q8. Upon cessation of input power, the storedpotential is released through the emitter (E) to the collector (C) oftransistor Q6 (Q6 is forward biased because the base (B) is a zeropotential and the voltage at capacitor C1 is impressed directly uponemitter (E)) to the gate (G) of MOSFET Q1 to open it and stop allcurrent passing from source (S) to drain (D), and simultaneously applyvoltage to gate (G) of N-channel MOSFET Q5 to close the switch betweensource (S) and drain (D) and drain all reverse EMF from motor 15. As isknown in the art, with respect to N-channel MOSFETs, applying voltage tothe gate (G) closes the path between source (S) and drain (D).

To further illustrate the operation, capacitor C1 is interposed theemitter of transistor Q6 and ground. The base of transistor Q6 isinterposed pin 53 and ground. The collector of transistor Q6 isinterconnected the gate of MOSFET Q1 through a diode D3 and the gate ofMOSFET Q5. The source of MOSFET Q5 is connected through a diode D4 tothe drain of MOSFET Q1 and motor pin 63 while the drain of MOSFET Q5 isgrounded. The gate of MOSFET Q5 is also grounded through a resistor R5.

When an input voltage of 12 volts DC is impressed on pin 53 (pin 53 ishigh, pin 55 is ground or low), diode D2 becomes forward biased allowingthe impressed voltage to charge capacitor C1 to a voltage level ofapproximately 10.7 volts. This charged voltage level at C1 is applied tothe emitter of transistor Q6. As the base of transistor Q6 is connectedto the input voltage at pin 53, there is no voltage or potential dropbetween the emitter and the collector of transistor Q5, and, this lackof a voltage drop reverse biases Q6 and prevents the collector fromemitting any voltage or current. With the voltage level at the collectorof transistor Q6 at zero, because Q6 is turned off, diode D3, interposedbetween the collector of transistor Q6 and the gate of MOSFET Q1 also isreverse biased and the voltage potential between the gate and the sourceof MOSFET Q5 is also essentially zero. As with all N-channel MOSFETs, azero or negative potential between the gate and source prevent anycurrent passage between the source and the drain. This is known in theart as having the MOSFET turned "off". This is the condition of thecircuit as motor 15 is operating. When the controller orders a cessationin voltage or power input to pin 53, instantaneous braking OCCURS.

When the input voltage at pin 53 is terminated, transistor Q6 becomesforward biased because, while the emitter remains at the 10.2 voltpotential level, the base potential drops to about zero thereby causingthe forward bias or potential drop between the emitter and the base oftransistor Q5 allowing the collector to accept voltage. Transistor Q6now is turned "on" and capacitor C1 begins to discharge its storedvoltage through the emitter. Note that there is no loss of voltage backto pin 53 because diode D2 is interposed between capacitor C1 and pin53, the emitter of transistor Q2 and current-limiting resistor R1. Thedischarging of the potential from capacitor C1 into the emitter oftransistor Q6 causes a current to flow from the collector throughresistor R5 to ground. This resistance develops a potential at the gateof MOSFET Q5 and simultaneously at the gate of MOSFET Q1. MOSFET Q1 isimmediately turned off so that further current does not pass from thesource to the drain (and vice versa) while simultaneously a path isopened between the source and the drain of MOSFET Q5 to ground out anyreverse EMF generated at pin 63. That EMF passes through diode D4 andacross the source to drain of MOSFET Q5 directly to ground. Accordingly,while the input voltage and associated current has been terminated atpin 53, the reverse EMF developed by motor 15 is immediately drainedthrough diode D4 and MOSFET Q6 to ground, thus removing both forward andreverse EMF from motor 15 and resulting in an immediate motor stop.Considering that motor 15 would be turning at 7,000 rpm that would bereduced to approximately 20 rpm by gear reduction transmission 19 andoutputted through driven shaft 33, in association with a substantialload placed upon motor 15 through the transmission and the frictionassociated with the needle valve and its assembled parts, motor 15 stopsinstantaneously resulting in extreme accuracy in placing the valve inthe position called for by the controller.

Should motor 15 be operated in the reverse direction, the voltage inputwould be to pin 55 while pin 53 would be at ground or low. In thatsituation, the voltage would begin to build on capacitor C2 that isdownstream from a diode D5. NPN transistor Q7 has its base connected topin 55 and its emitter connected between capacitor C2 and diode D5. Thecollector of transistor Q7 is connected to the gate of P-channel MOSFETQ3 through a diode D6 and to the gate of N-channel MOSFET Q8 that is inturn connected across resistor R6 to ground. Just as in the case whenthe voltage was impressed at pin 53 to drive motor 15 in the forwarddirection, impressing the voltage at pin 55 causes motor 15 to turn inthe reverse direction (as shown by a dotted arrow) and allows capacitorC2 to charge to approximately 10.7 volts. Again, because theemitter-to-base voltage of transistor Q7 is zero, Q7 is reverse biasedor turned off and no current is received at the collector so that thegate of MOSFET Q3 is zero allowing full current to pass between thesource (pin 55) through the drain of MOSFET Q3 to pin 65 of motor 15 andat the same time, the zero voltage at the gate of the N-channel MOSFETQ8 prevents it from passing any current from its source through itsdrain, or vice versa.

When the power to pin 55 is terminated by the controller, the voltage atthe base of transistor Q7 immediately drops to zero while the storedvoltage on capacitor C2 begins to increase the potential at the emitterof Q7 thereby forward biasing it and turning it on so that current isreceived at the collector to be passed immediately to the gate of MOSFETQ8 and through diode D6 to the gate of MOSFET Q3. The positive potentialbuildup at the gate of MOSFET Q3 terminates the current flow from sourceto drain and terminates the current passing to pin 65 while any back EMFis drained across a diode D7 to the source of N-channel MOSFET Q8. Thepositive of voltage appearing at gate of MOSFET Q8 is developed becauseof the resistance at R6 thereby turning on the source-to-drain flow ofback EMF through MOSFET Q8 to ground.

To describe the novel limit-switch protection of the circuit,normally-open, physically-activated limit-switch means 73 isinterconnected the voltage source, voltage-regulated, current-passingmeans 57 and voltage-actuated first switch means 69. It is preferredthat means 73 comprise limit switches SW1 and SW2, previously introducedherein.

To further illustrate the operation, starting with the situation wherevoltage is impressed at pin 53 and pin 55 is held at ground or low, andmotor 15 is operating in the forward direction, when the slide onpotentiometer 39 (see FIG. 2) reaches the normally-open limit switchSW1, it closes. When the contacts of switch SW1 are closed, the inputvoltage is applied directly to the gate of MOSFET Q5 and to the anode ofdiode D3. Resistors R7 and R8 are interconnected respectively acrosslimit switches SW1 and SW2 and to MOSFETS Q1, Q5, Q3 and Q8 to insurethe polarity remains constant for the MOSFETs. Diode D3 becomes forwardbiased which increases the voltage at the gate of MOSFET Q1 toapproximately 11.3 volts. This high-gate voltage forces MOSFET Q1 toturn off suddenly, and insures it will remain off. With the inputvoltage also applied directly to the gate, the gate-to-source voltagepotential of MOSFET Q5 turns it "on" allowing drain of reverse EMF frommotor 15 through diode D4 directly to ground. This direct path allowsfor a fast discharge of the EMF which forces the motor to stop quickly.

When the voltage is impressed at pin 55, making pin 53 low, motor 15turns in the opposite direction. Upon closing limit switch SW2, sourcevoltage (at pin 55) is immediately applied to the gate of MOSFET Q8 andto the anode of diode D6. The gate at MOSFET Q3 is accordingly increasedto approximately the source voltage which suddenly turns it off. Thissudden turn off of source voltage to motor 15 is accompanied by asimultaneous opening or turning MOSFET Q8 "on" allowing the back EMF tobe drained through diode D7 and across the source to drain of MOSFET Q8to ground.

A unique feature of this limit switch aspect of the control circuit isthat the limit switch remains open during all normal operation and onlycloses when it is to shut down the operation of motor 15. This is inmarked contrast to the prior art where the limit switch remains closedduring normal operation and only opens when the motor is to be stoppedbecause of the valve-turning mechanism reaching one of the presetlimits. In this unique, inventive control circuit, there are no loads onlimit switches SW1 and SW2 so that their lives are greatly extended.

Below is listed a table showing the relative values of the componentsdescribed herein.

                  TABLE                                                           ______________________________________                                        Resistors         MOSFETs                                                     R1     1/2 Watt, 1.6 Ohm                                                                            Q1     P-channel 1RF9520                                R2     1/4 Watt, 10 K Q3     P-channel 1RF9520                                R3     1/2 Watt, 1.6 Ohm                                                                            Q5     N-channel 1RFD-123                               R4     1/4 Watt, 10 K Q8     N-channel 1RFD-123                               R5                                                                            R6                                                                            R7     1.6 mW                                                                 R8     1.6 mW                                                                 Capacitors                                                                    C1     22 uF 25 V                                                             C2     22 uF 25 V                                                             Diodes                                                                        D1     1N4002                                                                 D2     1N4148                                                                 D3     1N4148                                                                 D4                                                                            D5                                                                            D6                                                                            D7                                                                            Transistors                                                                   Q2     PNP 2N4403                                                             Q4     PNP 2N4403                                                             Q6     PNP 2N4403                                                             Q7     PNP 2N4403                                                             ______________________________________                                    

Means 57 may also be in the form of an NPN transistor while means 59 mayinclude inductors, transducers, and variable resistors. Means 67 mayinclude batteries while means 71 may include other types of transistors.

While the invention has been described by reference to a particularembodiment thereof, those skilled in the art will be able to makevarious modifications to the described embodiment of the inventionwithout departing from the true spirit and scope thereof. It is intendedthat all combinations of elements and steps which perform substantiallythe same function in substantially the same way to achieve the sameresults are within the scope of this invention.

What is claimed is:
 1. A control circuit for precisely limiting thetravel of a valve driven by a reversible motor comprising:a)voltage-regulated, current-passing means for passing current from adirect voltage source to the motor, said current-passing means having adefault configuration such as to permit full current flow there-acrosswhen said voltage source is initially applied; b) normally-open,voltage-actuated switch means interconnecting the motor to ground; and,c) normally-open, physically-activated, limit-switch meansinterconnected the voltage source, said current-passing means and saidvoltage-actuated switch means that remains open during travel of thevalve between limits physically established in said circuit so that,upon reaching one of said limits, said limit-switch means is closed toprovide immediate voltage to turn off said current-passing means and toclose said switch means to immediately drain all forward and reverse EMFto ground and stop rotation of the motor.
 2. The control circuit ofclaim 1 wherein said voltage-regulated, current-passing means comprisesa voltage-dependent, switching transistor for coupling and uncouplingthe voltage source to the motor.
 3. The control circuit of claim 2wherein said voltage-regulated, current-passing means comprises a FETtransistor.
 4. The control circuit of claim 3 wherein saidvoltage-regulated, current-passing means comprises a P-channel MOSFET.5. A control circuit for precisely limiting the travel of a valve drivenby a reversible electric motor and for instantaneously braking rotationof the motor upon termination of power from a voltage sourcecomprising:a) voltage-regulated, current-passing means for passingcurrent from the voltage source to the motor, said current-passing meanshaving a default configuration such as to permit full current flow thereacross when said voltage source is initially applied; b) means forstoring a potential during circuit operation including a normally-open,voltage-actuated first switch means for isolating said stored potentialfrom the rest of said circuit during motor operation; c) normally-open,voltage-actuated second switch means interconnecting the motor toground; and, d) normally-open, physically-activated, limit-switch meansinterconnected the voltage source, said current-passing means and saidvoltage-actuated first switch means that remains open during travel ofsaid valve between limits physically established in said circuit sothat, upon reaching one of said limits, said limit-switch means isclosed to provide immediate voltage to turn off said current-passingmeans and to close said second switch means to immediately drain allforward and reverse EMF to ground and stop rotation of the motor.
 6. Thecontrol circuit of claim 5 wherein said means for storing a potentialduring circuit operation additionally includes a capacitorinterconnected the voltage source to the motor and ground.
 7. Thecontrol circuit of claim 5 wherein said means for storing a potentialcomprises a battery.
 8. The control circuit of claim 5 wherein saidvoltage-regulated, current-passing means comprises a voltage-dependent,switching transistor for coupling and uncoupling the voltage source tothe motor, said switching transistor having a default configuration suchas to permit coupling the voltage source to the motor during motoroperation.
 9. The control circuit of claim 8 wherein saidvoltage-regulated, current-passing means comprises a P-channel FETtransistor.
 10. The control circuit of claim 5 wherein saidnormally-open, voltage-actuated first switch means comprises anormally-conducting PNP transistor whose base and emitter areinterconnected the voltage source across a voltage drop means and saidemitter is interconnected said means for storing a potential so thatduring motor operations said transistor is non-conducting and, upontermination of the voltage source, a potential is fed to said emitterfrom said potential storage means to close said transistor and establisha current flow therethrough.
 11. The control circuit of claim 5 whereinsaid normally-open, voltage-actuated second switch means comprises avoltage-dependent, switching transistor having an open defaultconfiguration and, upon termination of the voltage source, receives apotential at the gate from said first switch means to close and passforward and reverse EMF from said motor to ground.
 12. The controlcircuit of claim 11 wherein said normally-open, voltage-actuated secondswitch means comprises a FET transistor.
 13. The control circuit ofclaim 5 further including a diode operably interposed the voltage sourceand said means for storing a potential during circuit operation toisolate said means for storing a potential from the circuit.
 14. Acontrol circuit for precisely limiting the travel of a valve driven by areversible electric motor and for limiting the current flow to themotor, to reduce current spikes at motor start up, comprising:a)voltage-regulated, current-passing means for passing current from adirect voltage source to the motor, said current-passing means having adefault configuration such as to permit full current flow there-acrosswhen said voltage source is initially applied; b) normally-open,voltage-actuated switch means interconnecting the motor to ground; c)normally-open, physically-activated, limit-switch means interconnectedthe voltage source, said current-passing means and said voltage-actuatedswitch means that remains open during travel of said valve betweenlimits physically established in said circuit so that, upon reaching oneof said limits, said limit-switch means is closed to provide immediatevoltage to turn off said current-passing means and to close said switchmeans to immediately drain all forward and reverse EMF to ground andstop rotation of the motor; d) voltage drop means interposed the voltagesource and said voltage-regulated, current-passing means responsive tothe current flow to the motor; and, e) variable-voltage output meansresponsive to the voltage drop developed in said voltage drop means forproviding a subsequently changing voltage for input to saidcurrent-passing means for limiting the current flow therethroughfollowing initial application of the voltage.
 15. The control circuit ofclaim 14 herein said voltage-regulated, current-passing means comprisesa normally conducting P-channel field effect transistor interconnectedthrough its gate to said variable-voltage output means.
 16. The controlcircuit of claim 15 wherein said P-channel field effect transistor is aP-channel MOSFET.
 17. The control circuit of claim 14 wherein saidvariable-voltage output means comprises a transistor interconnected itsemitter and base across said voltage drop means.
 18. The control circuitof claim 17 wherein said transistor is a PNP transistor.
 19. The controlcircuit of claim 14 wherein said voltage drop means comprises aresistor.
 20. The control circuit of claim 14 wherein saidvariable-voltage output means comprises a PNP transistor whose base isinterposed said voltage drop means and said voltage-regulated,current-passing means and the emitter is interposed the voltage sourceand said voltage drop means so that increasing current flow across saidvoltage drop means creates an increasing potential gradient between saidemitter and said base to cause said transistor to dynamically respondwith increasing voltage at the collector for input to saidcurrent-passing means.